Suprachiasmatic nucleus VIPergic fibers show a circadian rhythm of expansion and retraction

Aug 10, 2024Current biology : CB

Daily changes in the size of special nerve fibers in the brain's internal clock

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Abstract

VIPergic fibers in the mouse suprachiasmatic nucleus undergo a daily rhythm of expansion and retraction, with maximal branching during the day.

The study focused on neurons in the mouse suprachiasmatic nucleus (SCN) that express vasoactive intestinal polypeptide (VIP).
These VIPergic fibers exhibit a circadian rhythm in their structure, similar to what has been observed in fruit fly clock neurons.
The rhythm of fiber expansion and retraction persists even under constant environmental conditions.
Both male and female mice display this daily structural change in VIPergic fibers.
Circadian structural remodeling of clock neurons may be essential for regulating network properties and responses to environmental changes.

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In animals, overt circadian rhythms of physiology and behavior are centrally regulated by a circadian clock located in specific brain regions. In the fruit fly Drosophila and in mammals, these clocks rely on single-cell oscillators, but critical for their function as central circadian pacemakers are network properties that change dynamically throughout the circadian cycle as well as in response to environmental stimuli.In the fly, this plasticity involves circadian rhythms of expansion and retraction of clock neuron fibers.Whether these drastic structural changes are a universal property of central neuronal pacemakers is unknown. To address this question, we studied neurons of the mouse suprachiasmatic nucleus (SCN) that express vasoactive intestinal polypeptide (VIP), which are critical for the SCN to function as a central circadian pacemaker. By targeting the expression of the fluorescent protein tdTomato to these neurons and using tissue clearing techniques to visualize all SCN VIPergic neurons and their fibers, we show that, similar to clock neurons in the fly, VIPergic fibers undergo a daily rhythm of expansion and retraction, with maximal branching during the day. This rhythm is circadian, as it persists under constant environmental conditions and is present in both males and females. We propose that circadian structural remodeling of clock neurons represents a key feature of central circadian pacemakers that is likely critical to regulate network properties, the response to environmental stimuli, and the regulation of circadian outputs. 1, 2, 3 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14

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Suprachiasmatic nucleus VIPergic fibers show a circadian rhythm of expansion and retraction

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